The invention pertains to a continuous heat treating (e.g., sintering) process for the production of ceramics, as well as the resulting ceramic wherein these ceramics may be used as cutting inserts, nozzles, wear parts and the like.
In the past, ceramics have been produced using a batch process. These ceramics have included silicon nitride-based ceramics, SiAlON-based ceramics, alumina-based ceramics, zirconia-based ceramics, titanium nitride-based ceramics, titanium carbide-based ceramics, and titanium carbonitride-based ceramics. These ceramics have also optionally contained additives which included whiskers such as, for example, silicon carbide, titanium carbide and/or titanium nitride whiskers so as to provide for whisker-reinforced ceramics. These additives have also comprised zirconia, as well as the nitrides, carbides, borides and carbonitrides of titanium, hafnium and zirconium.
Referring specifically to SiAlON materials, SiAlON-based ceramics have been produced using a batch process. U.S. Pat. No. 4,563,433 to Yeckley et al. for a Ceramic Material and Method of Manufacture (which is hereby incorporated by reference herein) describes such a batch process wherein a plurality of green compacts are buried in a boron nitride/silicon nitride setting powder mixture in a graphite pot. The pot, setting powder and green compacts are placed in a graphite element resistance-heated batch furnace, and then subjected to a heat treatment (e.g., sintering) process. The result is a densified SiAlON-based ceramic made according to a batch process.
U.S. Pat. No. 5,382,273 to Mehrotra et al. for a Silicon Nitride Ceramic and Cutting Tools Made Thereof, U.S. Pat. No. 5,370,716 to Mehrotra et al. for a High Z SiAlON and Cutting Tools Made Therefrom and Method of Using, and U.S. Pat. No. 5,525,134 to Mehrotra et al. for a Silicon Nitride Ceramic and Cutting Tool Made Thereof (all of these patents are hereby incorporated by reference herein) each pertain to ceramics made via a batch process. Although the batch-processed ceramics have adequate physical properties and performance characteristics for applications such as cutting inserts, these ceramics still exhibit certain disadvantages.
Some SiAlON-based ceramics, e.g., cutting inserts, as well as silicon nitride-based ceramics (e.g., cutting inserts), must present a uniform appearance on their surface so as to be visually pleasing. During the batch process, a reaction layer forms on and near the surface of the cutting insert. In batch processing, this surface reaction layer typically has a nominal thickness between about 0.010 inches (0.254 millimeters [mm]) and about 0.015 inches (0.381 mm). This surface reaction layer causes a change in the color of the surface of the cutting insert so that a batch-processed SiAlON-based or silicon nitride-based cutting insert does not exhibit a uniform surface color or appearance. In order to obtain the desired uniform surface appearance, the surface of the batch-processed cutting insert must be ground at least 0.010-0.015 inches so as to remove the surface reaction layer. There would be a significant cost savings associated with a process that would eliminate the necessity for the above-described grinding step. There would also be a significant cost savings associated with a process that would reduce the amount of grinding necessary to achieve an acceptable surface appearance from the amount of grinding required for a batch-processed cutting insert.
The process for making a batch-processed ceramic part (e.g., a cutting insert) requires that the ceramic part be physically removed from the tray in which it was delubed, and then physically placed in the tray in which it is batch processed. It is apparent that this transfer step adds an additional step, as well as labor costs, to the overall batch process. It would be desirable to be able to provide a process in which the ceramic part remained in the same container or tray from the delubing operation through the sintering operation. Such an advantage would reduce the number of manufacturing steps and the amount of labor needed in the production of a ceramic part.
In one form thereof, the invention is a continuous process for the manufacture of a ceramic sintered compact wherein the process comprises the steps of: forming a green compact from a powder mixture comprising a first component comprising compounds which contain elements of silicon, aluminum, oxygen and nitrogen; and the powder mixture further comprising a second component comprising a compound of at least one element selected from the group consisting of yttrium, scandium, cerium, lanthanum and the metals of the lanthanide series, and the second component comprising between 0.1 and 10 weight percent of the powder mixture; heat treating the green compact wherein the heat treatment comprises: continuously passing the green compact through at least one heating zone so as to produce a sintered compact.
In another form thereof, the invention is a continuous process for the manufacture of a ceramic sintered compact wherein the process comprises the steps of: forming a green compact from a powder mixture of silicon nitride and one or more sintering aids; subjecting the green compact to a heat treatment wherein the heat treatment comprises continuously passing the green compact through at least one heating zone so as to form the sintered compact and wherein the heating zone is at a temperature greater than 1750xc2x0 C.; and the ceramic comprising a beta-silicon nitride phase and an intergranular phase.
In still another form thereof, the invention is a ceramic cutting insert which comprises a cutting insert body which has an as-molded rake face and an as-molded flank face wherein the rake face intersects with the flank face to form a cutting edge at the intersection. The cutting body has at least one of the as-molded rake face and the as-molded flank face does not have a surface reaction layer.
In still another form thereof, the invention is a ceramic cutting insert that comprises a cutting insert body which has a microstructure including pockets of a glassy phase. At least ninety percent of the pockets of the glassy phase have a major axis which is of a dimension of one micron or less.
In another form thereof, the invention is a sintered ceramic body which comprises a substrate that comprises a two-phase composite of alphaxe2x80x2-SiAlON and betaxe2x80x2SiAlON, and a glassy phase. The substrate presents a surface. The substrate has a surface region extending inwardly from the surface with a bulk region beneath the surface region. The surface region has a higher alphaxe2x80x2-SiAlON content than the bulk region.